They're different philosophies with slightly different, totally equivalent, and debatably the same, curriculum.
I personally think OMM helps so much with anatomy and think of it as applied anatomy and as fixed patient encounters. I'd argue it is an essential course as cadavers can only get you so far, it takes a long time to get comfortable with the human body, the amount of anatomical variation in humans is astounding and you need to see a lot of bodies to get a full feel for it. If there are any course deficiencies, I'd argue it comes from filler classes the first two years. We take the same licensing exams and most DOs end up competing with allos for ACGME residencies for the same jobs with the same pay.
DO schools are deficient in research (relative to MD schools) and some of the new ones don't have the best system for rotations. That speaks nothing of quality and just because USNWR adds points for research doesn't mean research helps with being a better physician. As someone who has worked in clinical labs with MD for years and non clinical labs with PhDs for years, my personal opinion is physicians without a PhD are under qualified to do research and we should let the PhDs do their thing, while we focus on the clinical applications and/or applied research, though I am in the minority on this.
As to why have MDs (or DOs) at all, it is about ensuring competition and an adequate supply of docs. The AMA acted as a guild, and frankly, a monopoly, unduly influencing the physician supply and restricting how docs could advertise their services. They didn't like DOs, so the AMA influenced legislation against them (and bought their schools out). But their was still a shortage of docs so Carib schools were set up. Now we have something like 1/3 of all docs as non AMGs which poses its own problems (especially for newer non AMG graduates and AMG students rotating with them).
This has stopped and the AMA is a weak organization, and has been for the past few decades, but having only one licensing organization is risky. Absolute power corrupts absolutely. Better to have two licensing organizations imo.